A galaxy is a collection of many millions of stars.
Galaxies A typical galaxy is a collection of many millions of stars. A galaxy also contains large amounts of gas, dust and other objects. All of the stars and other objects in a galaxy are held together by a strong force called gravity and move around within the galaxy. Entire galaxies also move about in space. Galaxies can be different shapes but most galaxies are spiral. Our galaxy is called the Milky Way. The Milky Way is a typical spiral galaxy.
Fig. 2 Our galaxy is called the Milky Way.
A solar system is made up of a star and all the objects moving around it.
Solar systems A solar system is made up of a star and the objects that move around it. Our solar system is made up of our sun and eight planets as well as moons, asteroids, comets, gas and dust.
Fig. 3 Our solar system. A planet
is a large round space object that circles a star in its own orbit.
Planets Planets are large round objects that travel around a star in a set path called an orbit. The orbit is usually either circular or elliptical (oval). A planet reflects the light from the star and does not generate its own light. Eight planets have been discovered in our solar system. All eight planets travel around the sun in different orbits. The planets closest to the sun are called the inner planets. The inner planets are made up mostly of rock. They are called:
l Mercury l Venus l Saturn l Earth l Uranus
Fig. 4 Earth’s orbit around the sun is not a perfect circle. It is an ellipse.
l Mars
The outer planets are large balls of gases with rings around them. They are called: l Jupiter
l Neptune 378
A planet is said to have cleared its own orbit because any smaller objects that were in its way have either been forced out of the solar system or have become part of the planet. Smaller space objects may become part of a planet because the planet’s gravity pulls them in.
